If you throw all that out the window and instead argue that it's the act of downloading a song which is infringement (which current copyright law does not support), then this becomes really easy. Each downloader becomes liable for a single copy (the one they downloaded). And an appropriate fine would be, say, 3x or 5x the cost of buying the song from a legitimate source. So about $3-$5 per song. Frankly I think that's a much more sensible approach to copyright enforcement than ruining people's lives and depriving them of Internet service because they shared some music files.
But I suspect the *AA is going to want their cake and eat it too, and want to assess hundred-thousand dollar judgments against downloaders as well. This is a slimy and illogical (should be illegal) tactic of turning n crimes into n^2 crimes. If 10 people share a file and each copyright violation costs $100, then there are a total of 9 illegal copies made, and the total damages should be $900. But by the *AA's nonsensical reasoning, each person is responsible for 9 counts of copyright violation, so each person should pay $900, resulting in $10,000 in damages awarded. The math simply doesn't add up - they'd be getting $10,000 in court awards when the law has determined that they've only suffered $900 in damages.
You can't have it both ways. Either one person is liable for all the copyright infringement and you can ruin them financially. Or each person is responsible for a single copyright infringement (the file they downloaded) and you can only fine them a few times what it would've cost to buy the file legitimately.